Ice layers at technical surfaces often cause serious safety risks and damages in technical applications. The ice formation at especially air-plane wings, ship decks and cables during the winter time are well known examples. The avoidance of this ice formation and the lowering of the adhesion between ice and surface have therefore been in focus for international research and development in the last few decades.
Hydrophobic surface properties are a known basis for the design of ice repellent coatings. A relative large amount of different coating systems have been developed and applied during the last 30 years. According to the definition of the work of adhesion, mainly water repellent materials with a very low surface tension like PTFE (Teflon) or Polyethylene have been chosen. In recent years, new silicone containing coatings like PDMS (Polydimethylsiloxane) were introduced for this purpose, as well.
For atmospheric ice, the problems could partly be solved by these coatings. But the live time, the mechanical stability, and also the stability of the anti adhesive properties are not satisfying expectations. Commercially available icephobic coatings are typically freeze-depressing due to the hydrophobic effect. Unfortunately, at certain super cooling, strong icing occurs at these coated surfaces, destroying the icephobicity. Challenges especially for air plane applications still exist.
Ice generation at cooled surfaces for refrigeration purposes or ice cream production shows similar challenges: the here desired and formed ice has to be removed by expensive mechanical scrapping devices or by short time heat input. The application of the above mentioned coatings has also been tried in this branch, but without success.
The latter being due to mainly three reasons, the change of properties of the coating during use, especially because of wear, the dramatically reduction of heat transfer due to the very low heat conductivity of these typically thick coatings, and the heavy decreasing ice forming rate because of the low wet ability of these surfaces.
On a normal treated, cooled heat exchanger surfaces, the crystallized ice is sticking with strong adhesion forces. According to the temperature gradients just above the cooled surface with the flowing brine, a flat, compact ice layer will be created. If not removed by, for example, mechanical scrapping devices, this ice layer may increase in thickness with time and result in a decreasing and finally stopping flow.
Existing developments on the basic of fluorinated organic coatings, cannot overcome this problem. The typical water repellent properties of fluorinated organic surfaces at room temperature are generally not transferable to ice repellent properties for the actual application.
Fluorinated coatings such as commercial types of PTFE, FEP but also fluorinated alkoxsilanes from different sol gel coating systems have been tested but appeared generally not convincing in their icephobicity. In the tests, these fluorinated coatings showed the highest hydrophobicity at contact angles of approximately 95 up to 115 degrees to water and water/freezing-depressant solutions. But this higher hydrophobicity had no visible influence on the surface-icing behavior. Based on the tests, it can be concluded that hydrophobic surfaces are not consequently also icephobic. Obviously, the distinct hydrophobicity of fluorinated coating systems is no longer effective when ice crystals with their strong polarity and directed dipole moments are formed at the surface. A dipole seems to be induced in the high electronegative fluorine resulting in secondary forces and sticking of the ice. Furthermore, mechanical interlocking of the ice crystals formed at the PTFE surface occurs because of the well-known porosity of the sintered PTFE surface.
Though existing silicone based lacquer coating types show good ice repellent properties, a very insufficient mechanical stability and very low heat conductivity of these coatings makes the application for the actual purpose impossible. The relative abrasive ice particles destroy these coatings already after short time. Also, the dramatically decreased ice nucleation rate is a further highly negative factor.
According to the disclosure in International patent application WO 00/06958, sol gel technology has been used for the production of a corrosion resistant hydrophobic coating which also appears to prevent ice formation on the surface of an evaporator. This condition is however only valid for surface temperatures that are very close to the freezing point. If the temperature of the surface is substantially below the freezing point, ice will form on the surface despite the hydrophobic conditions.